Training Manual - Potter Electric Signal Company, LLC

#3. Wet System With Maintained Excess Pressure
This type of system has a jockey or excess pressure pump that is maintaining a higher pressure on the system
side of the check valve than the supply pressure can be expected to reach. This excess pressure holds the clapper
of the valve down even when the city pressure fluctuates. This eliminates false alarms caused by water surges.
Fig. 5
Wet System With Excess Pressure (Pressure Drop)
Waterflow Alarm Installation
RISER TO
SPRINKLER HEADS
Two different methods utilizing two different waterflow devices can be used to monitor these types of systems.
Excess pressure systems do not require retards on the flow switches.
On both systems the excess pressure pump should be monitored with a supervisory pressure switch. This switch
should be set to trip when the pressure drops 10 psi below normal, this will prevent false alarms due to a drop in
pressure from a pump failure. A typical excess pressure system may maintain a system pressure of 120 psi. The
low pressure supervisory switch would be set to trip at 110 psi. High pressure should be monitored to prevent
damage to sprinkler system and long delay of a waterflow alarm.
PS100-2
WATERFLOW SWITCH
OPERATES ON
PRESSURE DROP
PS120-2
EXCESS PRESSURE
SUPERVISORY SWITCH
BVL
BLEEDER VALVE
(OPEN)
CLAMP TYPE
FITTING
A. Straight Riser With Maintained Excess Pressure
A pressure drop type waterflow device is connected to the system side of the check valve. Activation of a
sprinkler head in the system results in a loss of excess pressure, the pressure will drop down to whatever the
supply pressure is. The waterflow switch detects this drop in pressure and sends an alarm signal.
CHECK
VALVE
BVL BLEEDER
VALVE (OPEN)
EXCESS
PRESSURE
GAUGE
The low pressure supervisory switch is set to trip approximately 10 psi below the normal excess pressure.
The waterflow switch would be set to trip 10 psi below the supervisory switch. The trip point of the waterflow
switch must be higher than the water supply pressure can be expected to reach.
Note: All water control valves should be supervised per NFPA13 2010 8.16.1.1.2.1.
EXCESS PRESSURE
LINE VALVE (OPEN)
DRAIN VALVE
(CLOSED)
OPEN
JOCKEY
PUMP
DRAIN
PIPE
OSY CONTROL
VALVES (OPEN)
WATER
CHECK
VALVE
TAMPER
DEVICE
OSYSU
TAMPER
DEVICE
OSYSU
BALL DRIP VALVE
FIRE DEPT.
CONNECTION
(SIAMESE)
CITY WATER
SUPPLY
CHECK
VALVE
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